Contribution of Post-Rolling Surface Treatment to Corrosion Performance of Duplex and Austenitic Stainless Steel Reinforcing Bar

This study elucidates the complex relationship between stainless steel (SS) rebar surface treatment and corrosion resistance in high chloride environments using microscopy, analytical, and conventional electrochemical and novel corrosion microscopy techniques in a short- and long-term assessment. Previous studies on carbon steel reported that millscale defects (cracks and porosity) compromise corrosion resistance by allowing chloride ions to reach the base steel. This study examining three SS grades with four different surface type shows that their high Cr content improves compactness and reduces defectiveness of the inner (Fe-Cr) millscale layer, resulting in better corrosion resistance which varied with surface treatment methods, alloy composition, concrete composition, and chloride exposure method. Despite its defective outer (Fe) millscale layer, as-rolled (AR) bars showed comparable or better corrosion resistance than bars subjected to post-rolling treatment. Treating SS surface with mechanical blasting or acid pickling alone deteriorates corrosion performance due to embedment of blasting particles into rebar surface and/or the presence of loose residual millscale. Employing both shotblasting and acid pickling (SBPK) causes micropits to develop on the bar surface, which deteriorates corrosion resistance. Pore solution tests do not adequately reflect corrosion performance of millscales, as they exhibit high corrosion rate upon uniform exposure to chloride but show better performance in long-term mortar/concrete tests due to longer equilibration of the millscale and passive film with concrete’s high pH environment and non-uniform chloride exposure to rebar surface. Overall, this work shows that achieving consistent post-roll treatment or pristine SBPK surface between manufacturers and batches is a bigger challenge than achieving homogeneous millscale, making AR SS rebar a cost-effective corrosion resistant option for construction.